start discovery on service changed indications when CLCBs are busy

Previously, if all CLCBs were busy when a service
changed indication was received, discovery would be skipped.
This prevented handling subsequent service changed indications.

This commit queues the discovery request on a busy CLCB, ensuring that
service discovery is performed even when all CLCBs are busy.

Bug: 400828637
Bug: 394154968
Test: atest GattClientTest
Change-Id: Ia92596021b3f09a86cb905fd76a740b9a2b9bf8e